In general, power over Ethernet (PoE) refers to a technique for delivering power via Ethernet cabling to a powered device, which is an electronic device adapted to derive power from the Ethernet cabling and which is outlined in IEEE Std 802.3™-2005 clause 33. PoE eliminates the need for a separate power source for powered devices, simplifying installation. Powered devices may include voice over Internet protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and the like.
An unexpected loss of power to a powered device may cause loss of data, loss of a device's state, and so on. Moreover, an unexpected power loss event may cause problems with associated software. An unexpected loss of power may occur, for example, when an Ethernet cable is disconnected from the powered device. To perform a controlled shutdown (e.g. to allow time for storing a machine state and to initiate a controlled shutdown of associated circuitry), sufficient time to execute such processes is needed after detection of a power loss event.
The IEEE Std 802.3™-2005 standard specifies that a powered device should include an input filter capacitor of less than 120 nF and greater than 50 nF, and that an input power supply ripple at the input of an Ethernet cable to the powered device (at an RJ-45 input, for example), and at a switching frequency of 350 kHz, should be less than 100 mV. This input filter capacitor introduces a capacitive time constant that produces a voltage decay in response to a power loss event. The input filter capacitor initially discharges to provide a filtered voltage supply with a relatively linear decreasing slope, which may be difficult to detect relative to other types of power fluctuations. Even when a power source has been removed, the filter capacitor may prevent powered circuitry coupled in parallel with the capacitor from experiencing immediate power loss, since the capacitor discharges like a battery. Therefore, there is a need for enhanced power loss detection in powered electronic devices.